Scanning probe microscopy (SPM) is born with the invention of the scanning tunneling microscope. In brief, it aims at forming images of sample surfaces using a physical probe. SPM techniques rely on scanning such a probe, e.g. a sharp tip, just above a sample surface whilst monitoring interaction between the probe and the surface. An image of the sample surface can thereby be obtained. Typically, a raster scan of the sample is carried out and the probe-surface interaction is recorded as a function of position. Data are thus typically obtained as a two-dimensional grid of data points.
The resolution achieved varies with the underlying technique: atomic resolution can be achieved in some cases. Use can be made of piezoelectric actuators to execute motions with a precision and accuracy, at any desired length scale up to better than the atomic scale.
The two main types of SPM are perhaps the scanning tunneling microscopy (STM) and the atomic force microscopy (AFM). The invention of STM was quickly followed by the development of a family of related techniques (including AFM), which together with STM form the SPM techniques.
The interaction monitored in STM is the current tunneling between a metallic tip brought in very close proximity to a conducting substrate. The name arises from the quantum mechanical concept of tunneling. Quantum mechanical tunneling allows for particles to tunnel through a potential barrier which they would not surmount according to the paradigm of classical physics. Yet, in the quantum world, electrons are able to hop through the classically-forbidden space between the tip and the sample.
Imaging of the surface topology may then be carried out in one of two modes:                in constant height mode, wherein the tunnel current is monitored as the tip is moved parallel to the surface); and        in constant current mode, wherein the tunnel current is maintained constant as the tip is scanned across the surface and a deflection of the tip is measured.        
In AFM techniques, forces between the tip and the surface are monitored; this may be either the short range Pauli repulsive force (in contact-mode) or the longer range attractive force (in non-contact mode, merely van der Waals forces).
In both STM and AFM, the position of the tip with respect to the surface must be very accurately controlled (i.e. to within about 0.1 Å) by moving either the sample or the tip. The tip is usually very sharp—ideally terminating in a single atom at its closest point to the surface.
Probe tips used are typically made of platinum/iridium or gold. In this respect, two main methods for obtaining a sharp probe tip are known: acid etching and cutting. The first method involves dipping a wire end first into an acid bath and waiting until it has etched through the wire and the lower part drops away. The resulting tip can thus often be one atom in diameter at its end. An alternative and quicker method is to take a thin wire and cut it with convenient tools. Testing the tip produced via this method on a sample with a known profile will then indicate whether the tip is suitable or not.
The STM is the actual precursor to the AFM, developed by Gerd Binnig and Heinrich Rohrer in the early 1980s, a development that earned them the Nobel Prize for Physics in 1986. Binnig, Quate and Gerber invented the first AFM in 1986. Since then, a number of variants or improvements of SPM methods and devices have been disclosed.
For the sake of exemplification, U.S. Pat. No. 5,059,793 (A) provides a scanning type tunnel microscope in which a servo system for controlling the distance between the probe and the sample can be set in a proper condition irrespective of the surface condition of the sample. It further discloses a scanning type tunnel microscope capable of setting the starting position of the scanning operation for a desired scanning range to a desired position after the wide range scanning operation is effected without using a rough moving mechanism necessary for movement of the probe in a vertical direction so as to always correctly set the desired position and maintain the reliability of an enlarged image. In particular, a tunnel probe used as a metal probe having a sharp tip end can be supported on a bottom surface of a tube scanner. The tunnel probe is typically mounted to be supplied with a bias voltage by means of a 10-bit D/A converter. On the other hand, a sample is disposed on the top surface. A tunnel current flows in the sample when a preset bias voltage is applied thereto with the tunnel probe set as close as approximately 1 nanometer (nm) to the surface of the sample. The tunnel current flowing in the sample is supplied to a servo circuit, 12-bit A/D converter.
As another example, U.S. Pat. No. 5,546,375 (A) provides a method of manufacturing a fine tip for detecting a fine current or force. The method the steps of: (a) forming a recessed portion in a surface of a first substrate; (b) forming a peeling layer on said first substrate; (c) laminating a fine tip material on said peeling layer; (d) joining said fine tip on said peeling layer to a second substrate; and (e) performing a peeling on an interface between said peeling layer and said first substrate or between said peeling layer and said fine tip to transfer said fine tip onto said second substrate.
As still another example, U.S. Pat. No. 4,874,945 (A) discloses an electron microscope equipped with a scanning tunneling microscope.
Beside the sole patent literature, a number of publications are directed to STM and the manufacture of SPM probes. For example, in a paper entitled “SQUID Probe Microscope Combined With Scanning Tunneling Microscope”, Hayashi, T., Tachiki, M., Itozaki, H., Applied Superconductivity, IEEE Transactions on Volume 17, Issue 2, June 2007 Page(s): 792-795 (DOI 10.1109/TASC.2007.898557), a high TC SQUID probe microscope combined with a scanning tunneling microscope for investigation of samples at room temperature in air is described. A high permeability probe needle was used as a magnetic flux guide to improve the spatial resolution. The probe with tip radius of less than 100 nm was prepared by microelectropolishing. The probe was also used as a scanning tunneling microscope tip. Topography of the sample surface could be measured by the scanning tunneling microscope with high spatial resolution. The SQUID probe microscope image could be observed while keeping the distance from the sample surface to the probe tip constant.